An electric power system requires that aggregate electric power production and consumption must be matched instantaneously and continuously, and that all system elements should operate within acceptable limits. Unexpected loss of generating units or transmission lines, or errors in daily load forecast, result in sudden imbalances between electric power generation and consumption. Such imbalances lead to frequency deviations from the nominal frequency of the power system (e.g., 50 Hz in Europe and 60 Hz in the U.S.). This is problematic because generators may get disconnected by over- or under-frequency protection systems and cause even larger deviations leading to a system blackout. Loads, such as synchronous rotating machines, need to operate at constant speed (frequency), and therefore, large frequency deviations may result in the interruption of various manufacturing processes.
An energy storage system (ESS) can be an effective means of alleviating these known problems. The ESS can function as a supplier of balancing power and energy reserves. An ESS may absorb power from the grid when the actual frequency is above a defined frequency tolerance band thereby charging the storage device, and an ESS may provide power back to the grid when the actual frequency is below the frequency tolerance band, in that case discharging the storage device.
As the growing size and number of power outages demonstrate, energy storage systems (ESSs) can be crucial for preventing the tremendous losses associated with momentary or prolonged power failures. In addition, an amount of large poorly dispatched renewable power plants (RPP) such as wind and solar is growing. At the same time, the dispatchable share of generation is strongly decreasing. More power and energy reserves are needed to guarantee a stable and secure operation of power systems due to the intermittent nature of renewable power plants which increases the carbon footprint of the energy supply system. Usage of a central ESS(s) based on a specific storage technology is well known and is typically used for a specific targeted application.
For example, flywheels and some batteries are limited to high power applications where EnergyESS[W*hours]<PowerESS[W]*t[hours] (e.g., for primary frequency control t is usually in the range of 0.1-1 hours) while pumped-hydro, compressed-air and thermo-electric energy storage are most suitable for high energy applications where EnergyESS>PowerESS*t (e.g., dispatching of renewables generation, arbitrage (day/night energy trading) and load levelling where t is above 1 hour). The first mentioned group of ESS technologies may have response times of less than one second, whereas the ESS technologies that are suitable for high energy applications generally have slower response times of 10 seconds or more.
A hybrid-ESS is a combination of fast and slow responding modules based on different energy storage technologies. Such a combination allows Hybrid-ESS to cover a full spectrum of utility scale applications from seconds to hours. For example, a Hybrid ESS may consist of a flywheel and flow battery or combine a lithium-ion battery and thermo-electric energy storage. A Hybrid-ESS may be configured in several different ways and includes a central stationary system in which both fast and slow reacting energy storage components are at the same location, a distributed stationary system in which single/multiple fast and slow reacting energy storage component(s) are situated in different locations, and a distributed stationary/mobile system in which a large number of fast reacting energy storage component are mobile.
In the case of a distributed Hybrid-ESS, grid constraints, which impact system reliability, must be taken into account when the energy is exchanged between remote components. Using a large number of mobile fast reacting modules (e.g., plug-in hybrid electric or fully electrical vehicles) may resolve transmission constraints since electric energy is moved from one area to another by means of conventional roads.
For example, if a Hybrid-ESS is used for dispatching a wind power plant using a stationary centralized fast and slow reacting modules, the slow reacting module is used to smooth slow power output fluctuations and for level the loading through the transmission line which links the renewable power plant (RPP) with the transmission grid. The fast reacting module is used to smooth fast renewable power fluctuations (e.g., wind gusts) and for fast network control services (primary frequency control). A Hybrid-ESS yields more flexibility and, hence, contributes to the benefit of ESS owner in various forms:
(i) Extension of provided services/number of utility scale applications; and
(ii) Exchange energy between fast and slow reacting ESS components (in periods when the fast component is at or anticipated to reach soon the operating limits) for minimizing operating cost.
These benefits can be achieved via a coordinated, optimally controlled charging/discharging process of fast and slow modules of the Hybrid-ESS. This process is mainly based on tracking of respective actual States of Charge (SoC) of the Hybrid-ESS modules. The actual SoC indicates how much energy can be provided to or absorbed from the grid by a module. When the SoC of a fast reacting module reaches or is anticipated to reach an upper or lower boundary value, the control system initiates energy exchange between fast and slow modules depending on ESS model predictions, power system forecasts, system constraints and optimal control objectives.
In WO2008/039725, a system is described which provides automated means to generate power and distribute locally generated power to a multiplexed array of energy storage devices in a dynamic manner. The system, when utilizing dynamic algorithms, aims to deal with the complex demands of often conflicting energy storage device requirements and real-time demand loads in conjunction with dynamic switching between energy storage devices to enhance the performance and effectiveness that is beneficial to both the aggregate energy efficiency and the individual owner demands of each energy storage device.
U.S. 2004/0263116 describes a distributed ESS which aims to store electrical energy close to the point of use. These storage nodes can communicate with a central clearing entity to determine whether the nodes should buy energy for storage, provide energy to the user, or sell power back to the grid. The function will depend, for example, on the amount of energy stored in the node, the cost of the electrical energy and peak power, and the price of resold energy/power.
Existing ESS control systems use real-time measurements of controlled parameters, predefined reference values and a predetermined control regime to control the use of the ESS or Hybrid-ESS. Such systems can be somewhat limited in their ability to provide suitable control for a complex hybrid ESS.
In view of the above, the present disclosure provides a system and method which utilize a set of techniques that can provide improved control performance for a hybrid ESS operation.